Hi,
If your prestashop application running on linux server ,please check the permission of the folder where files are getting copied.
Basically when user upload image then image get copied to required folder,if that folder do not have apropriate permission then error might come.
PrestaShop Tutorials Videos [How to do Tasks]
https://www.prestashop.com/forums/topic/907438-prestashop-tutorials-videos-how-to-do-tasks/